1. Field of the Invention
This invention relates to a packing container and a container closure having a coating layer or a printing ink layer of an ultraviolet ray-curable resin on an outside surface, and more specifically, to a packaging container and a container closure having excellent curability, aesthetic beauty of an ultraviolet ray hardened coating, transferability of a container, heat-resistant treatability, retortability, and sanitary characteristics.
2. Prior Art
An ink or coating composition containing an ultraviolet-curable resin does not have a problem of the volatilization of a solvent in drying or baking. Furthermore, since a printed matter or a coated product does not need heating, these compositions are used in various applications or such a use have been investigated.
As examples of heretofore used ultraviolet ray-curable resin commpositions, a combination of an epoxy resin and a cationic ultraviolet ray polymerization initiator, a combination of urethane (meth)acrylate and an acrylate monomer, and an epoxy (meth)acrylate resin have been known.
Japanese technical Publication No. 7344/96 (Published on Jul. 3, 1996; Nippon Unicar Co.; developer: Eiji Ando and Kenichi Suzuki; Int.Cl. C09D163/00) describes a resin composition composed of an ultraviolet-curable resin composition comprising an aliphatic cyclic epoxy resin, a cationic polymerization initiator, and polydiorganosiloxane-polyoxyalkylene alternate block copolymer.
An ultraviolet-curable resin composition contains an photo-cationic-polymerizable epoxy-type monomer and an photo-cationic-polymerization initiator has an advantage of forming a cured coating by ultraviolet irradiation without the need of heating. However, the ultraviolet-cured coating to be formed has an insufficient degree of curing, and as a coating layer or a printing ink layer of a package container, it has an insufficient problem in transferability, heat-resistant treatability and retortability. In this packaging container, a smooth transferring operation of a container is essential in a production step and a filling step. If the degree of curing is low in the coating layer or in the printing ink layer or slidability is poor on the outer surface, the coating will be easily damaged by contact with another object or by collision with another object. Furthermore, to improve the preservability of the contents, hot filling, sterilization by pasteurizer and retort sterilization have been carried out, but known ultraviolet ray-curable resins which can withstand these heat treatments, especially retort sterilization operation, are very few.
The photo-cationic-curing catalysts are decomposed by ultraviolet rays and release Lews acid. This Lewis acid polymerizes an epoxy group, and for example, an aromatic iodonium salt, an aromatic sulfonium salt, an aromatic selenium salt and an aromatic diazonium salt are used. However, although these photo-cationic-curing catalysts have a satisfactory sensitivity to ultraviolet rays having a short wavelength of generally 280 nm or below, they do not have a sufficient sensitivity to ultraviolet rays having a long wavelength. They only can insufficiently satisfy a purpose of forming the coating layer or the printing ink layer of the containers by curing due to ultraviolet rays. The pigments contained in the coating layer or the printing ink layer absorb ultraviolet rays having a short wavelength, for example, titanium dioxide absorbs ultraviolet rays having a shorter wavelength than 380 nm.
Furthermore, the photo-cationic-curing catalysts have a problem of sanitary characteristics. Known photo-cationic-curing catalysts may have a toxicity, and an iodonium salt has a problem of irritability of a bodily part. Furthermore, antimonate has a problem because it is composed of a heavy metal salt.
Of course, these photo-cationic-curing catalysts may be used in combination with known sensitizers, but many of the sensitizers have a problem of toxicity. Such sensitizers might not show a sensitizing action on ultraviolet rays having a long wavelength. Furthermore, some sensitizer may have a strongly brown color and may color the cured coated film. Combinations of photo-cationic-curing catalysts showing satisfactory curing properties to ultraviolet rays on the long wavelength with sensitizers have not been known so much.